102 ILUNOIS BIOLOGICAL MONOGRAPHS [390 



all along the strobila from close behind the scolex to well mto the region showing 

 the median row of reproductive rudiments, and in such a manner that, in the 

 anterior part of the strobila at least, what is considered to be a primary seg- 

 ment, situated between the most prominent transverse furrows, becomes sub- 

 divided into 2, 4, 8, 16 and finally 32 divisions, each of the latter accommoda- 

 ting two reproductive rudiments. But it must be emphasized that this method 

 of formation of new segments is not strictly followed out, since as it passes 

 backward in development, the primary segment does not always contain 64 

 genital rudiments. In the first place, some secondarj' or even tertiary trans- 

 verse furrows become almost as prominent as the primary ones, and secondly, 

 there is at the same time considerable further subdivision not only of the peri- 

 pheral tissues but especially of the rudiments themselves. Close behind the 

 scolex the primary segments are very short (Fig. 55), the first six to ten being 

 divided only into two subdivisions in strobilae of moderate size, but into 

 three or four subdivisions in the largest chains. Farther back this pro- 

 cess of segmentation takes place gradually; but division is usually seen to 

 occur more readily and quickly in the anterior part of the primary segment or 

 of its major subdivisions, i.e., secondary or tertiary, than in its posterior part — 

 occasionally however the reverse being the case. Thus in general there is a 

 sort of dominance of the anterior end of the segment, which one might call a 

 zooid in the sense in which Child uses the word, over its posterior end 

 as regards metamerism. While this method of formation of segments 

 is further obscured by the fact that often one sees intercalated amiong primary 

 segments, showing these features well, others which seem to lag behind in 

 division and are hence younger, and that in much elongated strobilas it is still 

 more difficult to distinguish between primary and secondary transverse furrows, 

 owing to their being quite smoothed out especially medially, the whole plan 

 is sufficiently clear to warrant its being described with the definiteness here 

 given. Figures 56 and 57 will give a better idea perhaps of the whole method 

 of segmentation than this description. While in figure 55 the primary seg- 

 ments are indicated by asterisks, in figures 56 and 57 the whole drawing is in 

 each case that of a primary segment. Under the heading of the reproductive 

 system below it will be seen that in the mature portion of the strobila the most 

 prominent transverse furrows are described as coming approximately every 

 eighth or sixteenth genital segment. This is due to the fact that the secondary 

 and tertiary furrows, respectively those dividing the primary segments into 

 two then four parts, become quite as pronounced as the primary ones, thus 

 making it very difficult to follow this plan of segmentation beyond the region 

 of differentiation of the genital rudiments. 



At least three prominent longitudinal grooves run thruout the median and 

 posterior portions of the strobila on each surface, even cutting thru the posterior 

 borders in many places. Their course is not regular and they are accompanied 

 by numerous other shorter and m.ore irregular grooves, some of which, but not 

 aU, are due simply to lateral contraction of the segments. 



